1. Field of the Invention
This invention relates to an inkjet printing apparatus, and in particular, to an inkjet printing apparatus for detecting a defective ejection of an inkjet nozzle in a print head of a full line type.
2. Description of the Related Art
A printing apparatus of an inkjet system is known as an apparatus for printing. The apparatus is equipped with a print head which ejects ink from nozzles arranged corresponding to a head resolution and fixes the ink on a print paper, thus forming characters and images thereon. The print head used in the printing apparatus of such an inkjet system is equipped with an energy generator for generating energy supplied to the ink for ejecting the ink as droplets from ejection openings. The print head is provided with an ink flow passage communicated with the ejection opening and ink accommodating device for accommodating ink supplied to the energy generator through the ink flow passage.
However, in a case of the print head in the inkjet system having this principle and structure, for example, in a case of having not used the print head for a long period of time, there are some cases where ink vaporizes from the ejection opening, which is solidified inside the nozzle or dusts or the like enters into the nozzle to clog the ejection opening. In addition, there are some cases where, due to foreign objects attached to the vicinity of the ejection opening, ink can not be ejected or a printing position of the ink is shifted. Further, there are some cases where air bubbles are generated inside the nozzle due to some cause, with which the nozzle is filled, and therefore, ink is not ejected, that is, so-called non-ejections are generated.
Even if such a defective ejection is generated, since the print head can not detect the defective ejection, there are some cases where even if printing is performed, ink is in fact not ejected at all or a droplet-hitting position of the ink is shifted. In addition, since a printing region corresponding to a width of a print medium is required in the full line type printing apparatus, an elongated print head is generally necessary, but when the print head is elongated, the number of nozzles in use increases depending on a printing density or a printing width to increase a generation rate of the defective ejection due to paper powder of the print medium, dusts or the like.
The nozzle in a state of such a defective ejection causes degradation of a quality in a printing image, and therefore there are proposed various methods for detecting the nozzle of the defective ejection to reduce an adverse effect of such a nozzle on the printing image.
An inkjet printing apparatus and an inkjet printing method disclosed in Japanese Patent Laid-Open No. 2006-205742 reads out an image printed on a print medium by reading device and detects a defective ejection nozzle from the readout image. When the defective ejection nozzle is detected, control of image correction, a recovery operation of the defective ejection nozzle or replacement droplet hitting by a normal ejection nozzle is performed.
However, the aforementioned conventional technology has the following problem. In Japanese Patent Laid-Open No. 2006-205742, an ejection state of each nozzle in the print head is determined based upon dots of the ejected ink. Therefore, a test pattern is printed by the print head and the printed test pattern is read out by the reading device. However, this publication does not disclose an adverse effect of an image distortion at the time of reading out the test pattern by the reading device or a technique for overcoming the effect. There are some cases where, upon reading out the image by the reading device, the readout image is distorted due to generation of optical aberration. Caused by the effect of the image distortion by the aberration, in spite of the fact that droplets of ink are normally hit, a position of an analysis region is shifted, and therefore, there occurs a problem that a defective ejection determination or a defective ejection direction determination is made based upon the analysis on the region shifted from the droplet hitting position. In addition, there occurs a problem that a pattern of the neighboring nozzle is erroneously detected due to the position shift of the analysis region and therefore a droplet hitting control for replacement is performed for the erroneous nozzle. Particularly in a case of using the print head with a high printing density, since the patterns corresponding to the nozzle are densely formed, higher detection accuracy to the pattern position is required in the process of detecting the defective ejection.